/**
 * Copyright 2018 jianggujin (www.jianggujin.com).
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.jianggujin.modulelink.mvc.util.captcha;

import java.io.IOException;
import java.io.OutputStream;

public class JEncoder {
   private static final int EOF = -1;

   private int imgW, imgH;
   private byte[] pixAry;
   private int initCodeSize;
   private int remaining;
   private int curPixel;

   // GIFCOMPR.C - GIF Image compression routines
   //
   // Lempel-Ziv compression based on 'compress'. GIF modifications by
   // David Rowley (mgardi@watdcsu.waterloo.edu)

   // General DEFINEs

   static final int BITS = 12;

   static final int HSIZE = 5003; // 80% occupancy

   // GIF Image compression - modified 'compress'
   //
   // Based on: compress.c - File compression ala IEEE Computer, June 1984.
   //
   // By Authors: Spencer W. Thomas (decvax!harpo!utah-cs!utah-gr!thomas)
   // Jim McKie (decvax!mcvax!jim)
   // Steve Davies (decvax!vax135!petsd!peora!srd)
   // Ken Turkowski (decvax!decwrl!turtlevax!ken)
   // James A. Woods (decvax!ihnp4!ames!jaw)
   // Joe Orost (decvax!vax135!petsd!joe)

   int n_bits; // number of bits/code
   int maxbits = BITS; // user settable max # bits/code
   int maxcode; // maximum code, given n_bits
   int maxmaxcode = 1 << BITS; // should NEVER generate this code

   int[] htab = new int[HSIZE];
   int[] codetab = new int[HSIZE];

   int hsize = HSIZE; // for dynamic table sizing

   int free_ent = 0; // first unused entry

   // block compression parameters -- after all codes are used up,
   // and compression rate changes, start over.
   boolean clear_flg = false;

   // Algorithm: use open addressing double hashing (no chaining) on the
   // prefix code / next character combination. We do a variant of Knuth's
   // algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
   // secondary probe. Here, the modular division first probe is gives way
   // to a faster exclusive-or manipulation. Also do block compression with
   // an adaptive reset, whereby the code table is cleared when the compression
   // ratio decreases, but after the table fills. The variable-length output
   // codes are re-sized at this point, and a special CLEAR code is generated
   // for the decompressor. Late addition: construct the table according to
   // file size for noticeable speed improvement on small files. Please direct
   // questions about this implementation to ames!jaw.

   int g_init_bits;

   int ClearCode;
   int EOFCode;

   // output
   //
   // Output the given code.
   // Inputs:
   // code: A n_bits-bit integer. If == -1, then EOF. This assumes
   // that n_bits =< wordsize - 1.
   // Outputs:
   // Outputs code to the file.
   // Assumptions:
   // Chars are 8 bits long.
   // Algorithm:
   // Maintain a BITS character long buffer (so that 8 codes will
   // fit in it exactly). Use the VAX insv instruction to insert each
   // code in turn. When the buffer fills up empty it and start over.

   int cur_accum = 0;
   int cur_bits = 0;

   int masks[] = { 0x0000, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF, 0x01FF, 0x03FF, 0x07FF,
         0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF };

   // Number of characters so far in this 'packet'
   int a_count;

   // Define the storage for the packet accumulator
   byte[] accum = new byte[256];

   // ----------------------------------------------------------------------------
   JEncoder(int width, int height, byte[] pixels, int color_depth) {
      imgW = width;
      imgH = height;
      pixAry = pixels;
      initCodeSize = Math.max(2, color_depth);
   }

   // Add a character to the end of the current packet, and if it is 254
   // characters, flush the packet to disk.
   void char_out(byte c, OutputStream outs) throws IOException {
      accum[a_count++] = c;
      if (a_count >= 254)
         flush_char(outs);
   }

   // Clear out the hash table

   // table clear for block compress
   void cl_block(OutputStream outs) throws IOException {
      cl_hash(hsize);
      free_ent = ClearCode + 2;
      clear_flg = true;

      output(ClearCode, outs);
   }

   // reset code table
   void cl_hash(int hsize) {
      for (int i = 0; i < hsize; ++i)
         htab[i] = -1;
   }

   void compress(int init_bits, OutputStream outs) throws IOException {
      int fcode;
      int i /* = 0 */;
      int c;
      int ent;
      int disp;
      int hsize_reg;
      int hshift;

      // Set up the globals: g_init_bits - initial number of bits
      g_init_bits = init_bits;

      // Set up the necessary values
      clear_flg = false;
      n_bits = g_init_bits;
      maxcode = MAXCODE(n_bits);

      ClearCode = 1 << (init_bits - 1);
      EOFCode = ClearCode + 1;
      free_ent = ClearCode + 2;

      a_count = 0; // clear packet

      ent = nextPixel();

      hshift = 0;
      for (fcode = hsize; fcode < 65536; fcode *= 2)
         ++hshift;
      hshift = 8 - hshift; // set hash code range bound

      hsize_reg = hsize;
      cl_hash(hsize_reg); // clear hash table

      output(ClearCode, outs);

      outer_loop: while ((c = nextPixel()) != EOF) {
         fcode = (c << maxbits) + ent;
         i = (c << hshift) ^ ent; // xor hashing

         if (htab[i] == fcode) {
            ent = codetab[i];
            continue;
         } else if (htab[i] >= 0) // non-empty slot
         {
            disp = hsize_reg - i; // secondary hash (after G. Knott)
            if (i == 0)
               disp = 1;
            do {
               if ((i -= disp) < 0)
                  i += hsize_reg;

               if (htab[i] == fcode) {
                  ent = codetab[i];
                  continue outer_loop;
               }
            } while (htab[i] >= 0);
         }
         output(ent, outs);
         ent = c;
         if (free_ent < maxmaxcode) {
            codetab[i] = free_ent++; // code -> hashtable
            htab[i] = fcode;
         } else cl_block(outs);
      }
      // Put out the final code.
      output(ent, outs);
      output(EOFCode, outs);
   }

   // ----------------------------------------------------------------------------
   void encode(OutputStream os) throws IOException {
      os.write(initCodeSize); // write "initial code size" byte

      remaining = imgW * imgH; // reset navigation variables
      curPixel = 0;

      compress(initCodeSize + 1, os); // compress and write the pixel data

      os.write(0); // write block terminator
   }

   // Flush the packet to disk, and reset the accumulator
   void flush_char(OutputStream outs) throws IOException {
      if (a_count > 0) {
         outs.write(a_count);
         outs.write(accum, 0, a_count);
         a_count = 0;
      }
   }

   final int MAXCODE(int n_bits) {
      return (1 << n_bits) - 1;
   }

   // ----------------------------------------------------------------------------
   // Return the next pixel from the image
   // ----------------------------------------------------------------------------
   private int nextPixel() {
      if (remaining == 0)
         return EOF;

      --remaining;

      byte pix = pixAry[curPixel++];

      return pix & 0xff;
   }

   void output(int code, OutputStream outs) throws IOException {
      cur_accum &= masks[cur_bits];

      if (cur_bits > 0)
         cur_accum |= (code << cur_bits);
      else cur_accum = code;

      cur_bits += n_bits;

      while (cur_bits >= 8) {
         char_out((byte) (cur_accum & 0xff), outs);
         cur_accum >>= 8;
         cur_bits -= 8;
      }

      // If the next entry is going to be too big for the code size,
      // then increase it, if possible.
      if (free_ent > maxcode || clear_flg) {
         if (clear_flg) {
            maxcode = MAXCODE(n_bits = g_init_bits);
            clear_flg = false;
         } else {
            ++n_bits;
            if (n_bits == maxbits)
               maxcode = maxmaxcode;
            else maxcode = MAXCODE(n_bits);
         }
      }

      if (code == EOFCode) {
         // At EOF, write the rest of the buffer.
         while (cur_bits > 0) {
            char_out((byte) (cur_accum & 0xff), outs);
            cur_accum >>= 8;
            cur_bits -= 8;
         }

         flush_char(outs);
      }
   }
}
